The discovery, development, and use of antibiotics to prevent and cure bacterial infections is one of the great milestones of modern medicine (Queijo, Jon. Breakthrough!: How the 10 Greatest Discoveries in Medicine Saved Millions, From Ancient Molds to Modern Miracles: The Discovery of Antibiotics. Chapter 7, FT Press Science (Pearson Education, Inc.), New Jersey, pp. 138-160). The β-lactam class of antibiotics has been one of our safest and most effective antibacterial agents (Hannoinen, J; Vaali, K; Koski, P; Syrjanen, K; Laitinen, O; Lindevall, K; Westermarck, E. (2003) Enzymic degradation of a β-lactam antibiotic, ampicillin, in the gut: a novel treatment modality J Antimicrob Chemother 51, 361-365). Since their introduction, bacteria have developed resistance to these antibiotics (Wright, G O (2011) Molecular mechanisms of antibiotic resistance. Chem Commun 47, 4055-4061). Bacterial production of β-lactamases, enzymes that inactivate β-lactam antibiotics through the catalytic hydrolysis of their lactam ring, is the most important and prevalent mechanism of resistance (Fisher. J. F., Meroueh, S O and Mobashery, S (2005) Bacterial resistance to-lactam antibiotics: compelling opportunism, compelling opportunity. Chem Rev 105, 395-424). Today there are over 1500 β-lactamases reported (β-Lactamase Classification and Amino Acid Sequences for TEM, SHY and OXA Extended-Spectrum and Inhibitor Resistant Enzymes, http://www.lahey.org/Studies/, accessed 10 Mar. 2014 and Site Web Institut Pasteur β-lactamase enzyme variants, http://www.pasteur.fr/ip/easysite/pasteur/en/research/plates-formes-technologiques/pasteur-genopole-ile-de-france/genotyping-of-pathogens-and-public-health-pf8/β-lactamase-enzyme- variants/β-lactamase-enzyme-variants accessed 10 Mar. 2014). There are now enzymes that inactivate every known class of β-lactam antibiotics (Peleg, A Y; Hooper, D C (2010) Hospital-Acquired Infections Due to Gram-Negative Bacteria. N Engl J Med 362:19, 1804-1813)
There are two families of β-lactamases, serine and zinc-dependent, that are divided into three functional classes according to Bush and Jacoby (Bush, K; Jacoby, J A, (2010) Updated Functional Classification of -Lactamases. Antimicrob Agents Chemother 54, 969-976). Classes 1 and 2 are serine enzymes that catalyze hydrolysis of β-lactam rings via a covalent intermediate known as an acylenzyme and Class 3 enzymes which are Zn2+-dependent and catalyze hydrolysis of lactam rings via direct nucleophilic attack of an active site hydroxide anion.
Previous attempts to circumvent β-lactamase-mediated antibiotic resistance can be divided into two strategies. The first, adding bulky functional groups to β-lactam antibiotics to make them poor β-lactamase substrates while maintaining their antibacterial activities has largely been circumvented by bacteria as there are now β-lactamases that inactivate every known clinically-approved β-lactam on the market (Peleg, A Y; Hooper, D C (2010) Hospital-Acquired Infections Due to Gram-Negative Bacteria. N Engl J Med 362:19, 1804-1813).
The second strategy has been the use of β-lactamase inhibitors in combination with β-lactam antibiotics. There are three clinically approved serine-3-lactamase inhibitors that have been used for decades (Drawz, S M; Bonomo, R A, (2010) Three Decades of β-Lactamase Inhibitors. Clin Microbial Rev 23, 160-201). Today these compounds are increasingly ineffective at protecting antibiotics from degradation by β-lactamase catalysis as selective pressure has led to the evolution of several inhibitor-resistant β-lactamases including groups 1, 1e, 2br, and 2ber as well as some group 2d, 2de, 2df and 2f enzymes. There are two serine-β-lactamase inhibitors in later stages of clinical development, Avibactam and MK-7655 (Boucher, H. W.; Talbot, G. H.; Benjamin, D. K.; Bradley, J.; Guidos, R. J.; Jones, R. N.; Murray, B. E.; Bonomo, R. a; Gilbert, D. (2013) 10×′20 Progress—Development of New Drugs Active Against Gram-Negative Bacilli: An Update From the Infectious Diseases Society of America. Clin. Infect. Dis. 1-10; Shlaes, D. M. (2013) New β-lactam-β-lactamase inhibitor combinations in clinical development. Ann N. Y. Acad. Sci. 1277, 105-14; Buynak, J. D. (2013) β-Lactamase inhibitors: a review of the patent literature 2010-2013. Expert Opin. Ther. Pat. 1-13; AstraZeneca-Our pipeline, http://www.astrazeneca.com/Research/Our-pipeline-summary, accessed 26 Feb. 2013). However, there remains some question about whether Avibactam is effective in inhibiting group 2d, 2de, and 2df enzymes and data suggest its combination with ceftazidime is ineffective versus Acenitobacter spp and anaerobic organisms (Zhanel, G G; Lawson, C D; Adam, H; Schweizer, F; Zelenitsky, S; Lagace-Wiens, P R; Denisuik, A; Rubinstein, E; Gin, A S; Hoban, O J; Lynch, J P 3rd; Karlowsky, J A (2013) Ceftazidime-Avibactam: a Novel Cephalosporin/β-lactamase Inhibitor Combination. Drugs 73:2, 159-177)
All of the β-lactamase inhibitors, approved and in development, bind at β-lactamase-active sites and require the formation of a covalent intermediate with the enzyme before β-lactamase inactivation can occur. Since none of the group 3 metallo-β-lactamases form a covalent intermediate with their substrates or inhibitors, all of these inhibitors are ineffective against group 3 β-lactamases (Cornaglia, G; GiamarellouH; Rossolini, G M (2011) Metallo-β-lactamases: a last frontier for β-lactams? Lancet Infect Dis 11:5, 381-393).
Another drawback of these known β-lactamase inhibitors is that they have no intrinsic, potent antibacterial activities. Thus there is an important need for β-lactam antibiotic compounds that also possess potent and broad spectrum β-lactamase inhibiting properties.